1. Field of the Invention
A method and related communication device for a wireless communication system is disclosed, and more particularly to, a method and related communication device for handling message decoding associated with a handover in a wireless communication system.
2. Description of the Prior Art
A handover procedure can transfer a mobile device from a communication state to another communication state. The communication state change can correspond to a telecommunication system, a cell, or a frequency layer scale, and corresponding handover is known as an inter-radio access technology (RAT), an inter/intra-cell or an inter-frequency handover.
For the inter-RAT handover, an X RAT can use an X handover procedure to hand over a mobile device, known as UE (User Equipment) or MS (Mobile Station), to a Y RAT by sending an X handover message including Y RAT configuration information. The mobile device configures itself to fit system requirements of the Y RAT according to the received Y RAT configuration information. Mostly, the Y RAT configuration information is formed as a message conforming to the specification of the Y RAT and includes mobility management, signaling connection, security, and capability information. Common RATs include UMTS (Universal Mobile Telecommunications System), GSM (Global System for Mobile communications)/GPRS (General Packet Radio Service), GERAN (GSM/EDEG radio access network) Iu mode system, CDMA (Code Division Multiple Access) 2000, and LTE (Long Term Evolution) systems.
HANDOVER COMMAND, PS (Packet-Switched) HANDOVER COMMAND and DTM (DUAL TRANSFER MODE) HANDOVER COMMAND messages are provided to initiate an inter-RAT handover to the GSM/GPRS mode system for CS (Circuit-Switched) or PS type connection transfer. The HANDOVER COMMAND message is a GSM RR (Radio Resource) message seen as a CS-type message, and the PS HANDOVER COMMAND and DTM HANDOVER COMMAND messages are GPRS (General Packet Radio Service) RLC/MAC (Radio Link Control/Medium Access Control) control messages seen as PS-type messages. The UE is equipped with a GSM RR message decoder for decoding the HANDOVER COMMAND message and a GPRS RLC/MAC control message decoder for decoding the PS HANDOVER COMMAND and DTM HANDOVER COMMAND messages.
For the inter-RAT handover from the UMTS to the GSM/GPRS system, a HANDOVER FROM UTRAN COMMAND message for initiating the inter-RAT handover can include one of the HANDOVER COMMAND, PS HANDOVER COMMAND and DTM HANDOVER COMMAND messages. A UE with CS and PS services in a UTRAN (UMTS Terrestrial Radio Access Network) receives a HANDOVER FROM UTRAN COMMAND message when a handover attempt for the UE is made by the UTRAN. However, the UE cannot know which GSM message is included in the received HANDOVER FROM UTRAN COMMAND message. Since the UE has no way to know a switched type (the CS or PS type) of the GSM message, errors in decoding the GSM message occur if the UE uses a wrong decoder. For example, if the UE uses the GSM RR message decoder to decode a GPRS RLC/MAC control message, the decoding error occurs. In this situation, the UE determines that the HANDOVER FROM UTRAN COMMAND message is invalid and then sends a HANDOVER FROM UTRAN FAILURE message to the UTRAN. Thus, lack of knowing the identity of the GSM message can cause the handover failure.
The inter-RAT handover from the LTE to the GSM/GPRS system has similar problem. When a UE in an E-UTRAN has a voice service (VoIP call) on going, the E-UTRAN can perform a PS handover to the GPRS/GERAN Iu mode system to continue the voice service or perform a SRVCC (Single Radio Voice Call Continuity) handover to the GSM/GERAN Iu mode system so that the UE can reuse the CS system. For initiation of the handover, the E-UTRAN sends to the UE a MobilityFromEUTRACommand message that may include either PS HANDOVER COMMAND or HANDOVER COMMAND message. However, the UE cannot know which of GSM or GPRS RLC/MAC message is included. The decoding errors can occur and thereby causes failure of the handover.